ON THE SUN'S HEAT. 413 



be 51,200 C., and the average translational 

 velocity of the molecules 67 kilometres per 

 second, being VT f IO ' 2 > tne velocity acquired 

 by a heavy body falling unresisted from the outer 

 boundary (of 40 times the radius of the earth's 

 orbit) to the centre of the nebulous mass. 



The gaseous nebula thus constituted would in the 

 course of a few million years, by constantly radiat- 

 ing out heat, shrink to the size of our present sun, 

 when it would have exactly the same heating and 

 lighting efficiency, but no notion of rotation. 



The moment of momentum of the whole solar 

 system is about eighteen times that of the sun's 

 rotation ; seventeen-eighteenths being Jupiter's and 

 one-eighteenth the sun's, the other bodies being 

 not worth taking into account in the reckoning of 

 moment of momentum. 



Now instead of being absolutely at rest in the 

 beginning, let the twenty-nine million moons be 

 given each with some small motion, making up in 

 all an amount of moment of momentum about a 

 certain axis, equal to the moment of momentum 

 of the solar system which we have just been con- 



